In particular, it applies to in-depth extended field microscopic optical image formation carried out at a high speed and especially to microelectronics inspection.
Generally speaking, in order to carry out scanning confocal optical microscopy, the procedure is as follows: a luminous beam is formed which is directed towards a focussing system.
By means of this focussing system, the beam is focussed onto the object to be studied.
By means of a separating plate, a light beam reflected by the object to be examined is sent to a system for the digital detection, analysis and processing of the detected signal.
The light intensity sent by the separating plate is detected and the detected signals are digitally analysed and processed.
A scanning of the luminous beam is carried out on the object to be studied, either by moving the luminous beam with the object being fixed, or by moving the object with the beam being fixed.
A known device enabling this method to be implemented is diagrammatically shown on FIG. 1.
A luminous beam delivered by a monochromatic luminous source 10, such as a laser, is spatially filtered and focussed by focussing means 12, focussing being effected with, for example, a lens, and filtering being effected with, for example, a diaphragm. This makes it possible to obtain an intensity uniformly distributed over the section of the beam. The beam is then refocussed onto the object 16 to be studied by focussing means 14, such as a lens. The beam is reflected by the object to be examined and sent back by a separating plate 18 to a detection system 20 connected to a system for the digital analysis and processing of the signals detected. The detection system 20 comprises a diaphragm 21 placed at a conjugated point of the focal point of the focussing means 14. The image obtained after digital processing represents the reflectivity variations of the object 16 to be examined on microscopic scale. The diaphragm 21, whose aperture is, for example, several tens of microns, makes it possible to avoid detection of the light derived from the non-focussed beams concerning the object to be examined. The effect of defocussing concerning the formation of images is shown on FIG. 2.
F0 is referred to as the focussing point of the beam after passing through the focussing means 14.
When the object 16 is placed inside the focal plane of the focussing means 14, the conjugated point of the point F0 is the point F'0 situated inside the plane of the diaphragm 21. The dimension of the luminous spot inside this plane is then minimal and the energy collected by the detection system 20 is maximal.
When the object is moved away from the focal plane of the focussing means 14, the image F'1 of the luminous beam reflected by the object is distanced from F'0. The luminous spot inside the plane of the diaphragm 21 is enlarged and the luminous energy collected by the detection system 20 is much less than it was previously.
In the presence of such a diaphragm 21, the microscope is used in the actual confocal mode. The diaphragm 21 enables a gain in the resolution of the final image of approximately a factor of 1.4 to be obtained with respect to the resolution of an image obtained without using a diaphragm.
The means used to obtain the scanning of the beam on the object 16 are not shown on FIG. 1.
In this type of device, the field depth is very small, namely in the order of from 0.3 to 0.5 .mu.m.
On the basis of using the device shown on FIG. 1, it is possible to carry out extended field in-depth optical image formation. It merely requires that cuts be made at successive distant altitudes of, for example, 0.5 .mu.m. In a known way, a scanning is made of the object 16 at an altitude z1 and then the object 16 is moved into the axis z of the device to an altitude z2 where a new scanning is carried on, and so on.
This method is described in section 5, page 123 of the book entitled "Theory and Practice of Scanning Optical Microscopy" written by Tony Wilson, Colin Sheppard, Academic Press in 1984.
The field depth obtained is then limited by the number of successive cuts made.
This method presents the drawback of requiring several acquisitions of successive images, which considerably reduces the optical image formation speed of the microscope. According to the known devices currently used, the formation of the image from a cutting of the object 16 (field depth 0.5 .mu.m) may take up to 2 s. In the case of a synchronized device with a picture monitor, the image formation time is the scanning time of a frame of the picture monitor. These times must be multiplied by the number of the desired successive images corresponding to the desired extended field depth.
The present invention makes it possible to carry out in-depth scanning optical microscopy of the extended field in a single scanning of the object 16 to be examined by the beam. Sequential acquisition is avoided and thus the acquisition speed of a complete image is considerably reduced.